Re: [avr-libc-dev] C11 Atomics and GCC libatomic

[David Brown]

On 26/06/16 17:15, Georg-Johann Lay wrote:
> There are quite some standard insns for atomics known to gcc, cf.
> "atomic_" insns in
>
> http://gcc.gnu.org/onlinedocs/gccint/Standard-Names.html
>
> This can avoid overhead of function calls.  But I still don't see the
> great advantage of using stdatomic over the "classical" util/atomic.h
> from avr-libc.  For C++ we don't have portability because there is no
> libstdc++ generated for avr, not even libsupc++...
>
> Johann

The "classical" atomic locks in util/atomic.h are for constructing 
critical sections - sections of code that can only be accessed by one 
context of execution at a time.  The stdatomic C11 and C++ atomics are 
for atomic data - variables that are accessed atomically regardless of 
contexts.  These are two different ways of looking at atomic accesses 
and correct interrupt or multi-threaded programming.  While either can 
be used to implement the other, they do not replace each other - the 
best choice depends on the type of code and the type of problem to be 
solved at the time.

Additionally, it is always good to implement standard solutions - it 
makes it a lot easier to write code that can be portable between 
different architectures.  So implementing the C11 and C++11 atomics 
helps make avr-gcc a better tool.

avr-gcc supports a fair amount of C++, even though there are many parts 
missing (last I looked, which was a good while ago, support for 
exceptions was missing).  People use C++ on the avr, and it makes a lot 
of sense to support as many features of the latest C++ standards as can 
conveniently be done.  Of course there are many things in C++ that would 
be disproportionately hard to support on the AVR, and are unlikely to be 
much used - but if C++11 atomics can be supported "for free" by Jacob's 
functions added to libatomic to support C11 atomics, then that is a 
great thing.

mvh.,

David